Glass manufacturing has a long history dating back to ancient Mesopotamia. Since then, the process of making glass and glass related products has developed into a complex mix of formulation chemistry, metallurgy, material science, engineering and art. Gorilla® Glass, a product of Corning Incorporated, DragonTrail™ Glass, by Asahi Glass Company, and Xensation® Cover Glass by SCHOTT illustrate some of the more recent advances made in glass strengthening technology.
Ion-exchanged glasses were first described by Kistler (1962) for soda-lime glass. This work was followed by Nordberg in 1964 for an alkali-alumina-silica glass and for an alkali-zirconia-silica glass. “Chemcor” was developed in 1960 by Corning Inc. and had few applications beyond windshields for race cars. However, the application environment for strengthened glasses has changed dramatically with the advent of personal electronic devices, video games, and touch technology. Most smartphones, tablets, and laptops are covered with one of these glasses, which exhibit excellent mechanical fracture resistance while maintaining good thermal and optical properties. The composition of “modern” ion-exchanged glasses is a trade-secret, however, its post-glass-production processing utilizes ion-exchange to replace small ions, such as lithium and/or sodium, with larger ions, such as sodium, potassium, or even rubidium, which results in a stronger, more durable final glass product.
Glass production and composition are inextricably tied together. A very simple glass formulation is pure silica. It contains just one ingredient: SiO2. Although silica itself forms an excellent glass with a wide range of applications, its use in everyday applications is prohibitively expensive due to its extremely high melting temperature (>1700° C.). This negatively impacts the cost to build continuous melters that can withstand the high temperature needed to reduce the viscosity of the glass below about 200-500 poise, resulting in prohibitively high capital and operating costs. Therefore, silica glasses require the addition of a flux to reduce the processing temperature to within practical limits, e.g. <1700° C. The most common fluxes are alkali and alkaline earth oxides. Most commercial glasses contain soda (Na2O) and lime (CaO), including those used for containers and window glasses. However, while the addition of fluxes to silica leads to decreased cost for the glass manufacturing, the addition of large amounts of alkali oxide results in degradation of the glass properties, in particular degradation in mechanical, chemical and optical properties. This degradation is particularly prevalent in glass substrates for liquid crystal displays that must be alkali-free. Low-cost manufacturing glasses such as soda-lime cannot compete with the new active matrix liquid crystal display (AMLCD) substrates available in the market. Some of the loss in glass physical properties can be countered by the addition of property modifiers, which include the alkaline earth oxides and transition metal oxides for some applications.
In an effort to find a more cost effective route to a high silica glass formulation, Hood and Nordberg, from Corning Glass Works, developed the process to produce Vycor® and is described in U.S. Pat. No. 2,106,744, U.S. Pat. No. 2,221,709, and U.S. Pat. No. 2,286,275, each of which is incorporated by referenced in their entirety. Vycor® is a glass with a composition that is 96% SiO2, 3% B2O3, 0.4% Na2O, and less than 1% Al2O3 and ZrO2. The high silica content provides very high temperature and thermal shock resistance. However, unlike pure fused silica, it can be manufactured into a variety of shapes, and manufacturing does not involve extremely high temperatures. The Hood and Nordberg process is a multi-step process that first creates a relatively soft alkali borosilicate glass. This material is then heat treated, which causes it to separate into two distinct glassy phases with differing chemical compositions. One phase is rich in alkali and boric oxide, this phase being acid soluble. The other phase is mostly insoluble silica. After the heat treatment, the two-phase “workpiece” is immersed in a hot, dilute acid solution. The soluble alkali phase is slowly dissolved, leaving behind a porous high-silica skeleton. The silica phase is then heated to above 1200° C., which consolidates the porous structure into a final dense, nonporous, high silica glass (Vycor®). The finished material resulting from this process is referred to in the industry as “reconstructed glass.”
Another abundant raw material in the earth's crust is alumina (Al2O3). Sapphire is the single crystal form of alumina, which has remarkable mechanical, optical, and electrical properties. Like silica, alumina has a very high melting point (>2000° C.), is very hard and is completely water insoluble. It is also an excellent electrical insulator and has a high thermal conductivity. In 1966, GE produced an alumina ceramic (with some crystalline content) called Lucalox™ and began selling the material in transparent alumina light bulbs. To make high alumina containing glasses, alumina is usually mixed with other oxides. For example, alkali aluminoborate glasses are commonly used for the production of reinforcing glass fiber (E-glass). However, a typical E-glass composition is about 57 mol % silica, 8.8 mol % alumina, 19.6 mol % CaO, 6.1 mol % B2O3, with the remainder consisting of MgO, Na2O, K2O, Fe2O3, TiO2 and F.
Although examples of high silica glasses and alumina ceramics are available, there is still a need for a low temperature (<1700° C.) production method for the manufacture of high alumina containing glasses (e.g. >35 mol %) with simpler formulations.